Untangling the quantum entanglement behind photosynthesis

The future of clean green solar power may well hinge on
scientists being able to unravel the mysteries of photosynthesis,
the process by which green plants convert sunlight into
electrochemical energy. To this end, researchers with the U.S.
Department of Energy (DOE)'s Lawrence Berkeley National Laboratory
(Berkeley Lab) and the University of California (UC), Berkeley have
recorded the first observation and characterization of a critical
physical phenomenon behind photosynthesis known as quantum
entanglement.

Previous experiments led by Graham Fleming, a physical chemist
holding joint appointments with Berkeley Lab and UC Berkeley,
pointed to quantum mechanical effects as the key to the ability of
green plants, through photosynthesis, to almost instantaneously
transfer solar energy from molecules in light harvesting complexes
to molecules in electrochemical reaction centers. Now a new
collaborative team that includes Fleming have identified
entanglement as a natural feature of these quantum effects. When
two quantum-sized particles, for example a pair of electrons, are
"entangled," any change to one will be instantly reflected in the
other, no matter how far apart they might be. Though physically
separated, the two particles act as a single entity.

"This is the first study to show that entanglement, perhaps the
most distinctive property of quantum mechanical systems, is present
across an entire light harvesting complex," says Mohan Sarovar, a
post-doctoral researcher under UC Berkeley chemistry professor
Birgitta Whaley at the Berkeley Center for Quantum Information and
Computation. "While there have been prior investigations of
entanglement in toy systems that were motivated by biology, this is
the first instance in which entanglement has been examined and
quantified in a real biological system."

The results of this study hold implications not only for the
development of artificial photosynthesis systems as a renewable
non-polluting source of electrical energy, but also for the future
development of quantum-based technologies in areas such as
computing - a quantum computer could perform certain operations
thousands of times faster than any conventional computer.

"The lessons we're learning about the quantum aspects of light
harvesting in natural systems can be applied to the design of
artificial photosynthetic systems that are even better," Sarovar
says. "The organic structures in light harvesting complexes and
their synthetic mimics could also serve as useful components of
quantum computers or other quantum-enhanced devices, such as wires
for the transfer of information."

What may prove to be this study's most significant revelation is
that contrary to the popular scientific notion that entanglement is
a fragile and exotic property, difficult to engineer and maintain,
the Berkeley researchers have demonstrated that entanglement can
exist and persist in the chaotic chemical complexity of a
biological system.

"We present strong evidence for quantum entanglement in noisy
non-equilibrium systems at high temperatures by determining the
timescales and temperatures for which entanglement is observable in
a protein structure that is central to photosynthesis in certain
bacteria," Sarovar says.

Sarovar is a co-author with Fleming and Whaley of a paper
describing this research that appears on-line in the journal
Nature Physics titled "Quantum entanglement in
photosynthetic light-harvesting complexes." Also co-authoring this
paper was Akihito Ishizaki in Fleming's research group.

Green plants and certain bacteria are able to transfer the
energy harvested from sunlight through a network of light
harvesting pigment-protein complexes and into reaction centers with
nearly 100-percent efficiency. Speed is the key – the
transfer of the solar energy takes place so fast that little energy
is wasted as heat. In 2007, Fleming and his research group reported
the first direct evidence that this essentially instantaneous
energy transfer was made possible by a remarkably long-lived,
wavelike electronic quantum coherence.

Using electronic spectroscopy measurements made on a femtosecond
(millionths of a billionth of a second) time-scale, Fleming and his
group discovered the existence of "quantum beating" signals,
coherent electronic oscillations in both donor and acceptor
molecules. These oscillations are generated by the excitation
energy from captured solar photons, like the waves formed when
stones are tossed into a pond. The wavelike quality of the
oscillations enables them to simultaneously sample all the
potential energy transfer pathways in the photosynthetic system and
choose the most efficient. Subsequent studies by Fleming and his
group identified a closely packed pigment-protein complex in the
light harvesting portion of the photosynthetic system as the source
of coherent oscillations.

In this new study, a reliable model of light harvesting dynamics
developed by Ishizaki and Fleming was combined with the quantum
information research of Whaley and Sarovar to show that quantum
entanglement emerges as the quantum coherence in photosynthesis
systems evolves. The focus of their study was the
Fenna-Matthews-Olson (FMO) photosynthetic light-harvesting protein,
a molecular complex found in green sulfur bacteria that is
considered a model system for studying photosynthetic energy
transfer because it consists of only seven pigment molecules whose
chemistry has been well characterized.

"We found numerical evidence for the existence of entanglement
in the FMO complex that persisted over picosecond timescales,
essentially until the excitation energy was trapped by the reaction
center," Sarovar says.

"This is remarkable in a biological or disordered system at
physiological temperatures, and illustrates that non-equilibrium
multipartite entanglement can exist for relatively long times, even
in highly decoherent environments."

The research team also found that entanglement persisted across
distances of about 30 angstroms (one angstrom is the diameter of a
hydrogen atom), but this length-scale was viewed as a product of
the relatively small size of the FMO complex, rather than a
limitation of the effect itself.

"We expect that long-lived, non-equilibrium entanglement will
also be present in larger light harvesting antenna complexes, such
as LH1 and LH2, and that in such larger light harvesting complexes
it may also be possible to create and support multiple excitations
in order to access a richer variety of entangled states," says
Sarovar.

The research team was surprised to see that significant
entanglement persisted between molecules in the light harvesting
complex that were not strongly coupled (connected) through their
electronic and vibrational states. They were also surprised to see
how little impact temperature had on the degree of
entanglement.

"In the field of quantum information, temperature is usually
considered very deleterious to quantum properties such as
entanglement," Sarovar says. "But in systems such as light
harvesting complexes, we see that entanglement can be relatively
immune to the effects of increased temperature."